1. Field of the Invention
The invention relates to an electrical transducer using a two-wire process, with a quality measurement sensor. An analog end stage is connected downstream of the sensor, the sensor and the analog end stage being connected to one another via an analog measurement signal transmission path, and in cooperation with a with a processor circuit, the end stage converts the output signal of the sensor into an impressed output current with a magnitude which is a measurement of the quantity to be measured. The electronic transducer can also be controlled with a processor circuit. In addition, the invention relates to a process for indicating a the measured value with an output current which is proportional to the measured value. The measurement is performed with an electrical transducer, the transducer having a sensor, an analog end stage which is connected downstream of the sensor, and a processor circuit, the analog end stage converting the output signal of the sensor into an impressed output current with a level which is a measure of the quantity to be measured, the electrical transducer capable of being controlled using the processor circuit.
2. Description of Related Art
Electrical transducers using the two-wire process are known, for example, as pressure transmitters. The sensor which has been integrated into the transducer generally has, besides the actual sensor element, a signal conditioning unit. The sensor element can be designed both for contact measurement and also for proximity measurement. Using the sensor element and the signal conditioning unit, which is connected downstream of the sensor element, the quantity to be measured is converted into an electrical output signal which is proportional, and generally linearly proportional, to the quantity to be measured, for example, a DC voltage signal or a direct current signal. In the analog end stage which is connected downstream of the sensor, for example, to a controllable power source, the output signal of the sensor is converted into an impressed output current which represents the output signal of the electrical transducer. Here the magnitude of the output current represents a measure of the quantity to be measured, for example, a pressure which is to be measured.
The output current is generally fixed within the range from 0 to 20 mA or from 4 to 20 mA, with an output current of 0 or 4 mA representing the starting point and an output current of 20 mA representing the end point of the measurement range. If the electrical transducer is, for example, a pressure transmitter with a measurement range from 0 to 100 bar, at a pressure of 0 bar measured by the sensor the pressure transmitter delivers an output current of 0 or 4 mA, while at a pressure of 100 bar measured by the sensor, the output current of the pressure transmitter is 20 mA. The ratio of the measured pressure to the delivered output current is thus linear, so that an output current of 0 or 4 mA corresponds to zero percent of the measurement range and an output current of 20 mA corresponds to one hundred percent of the measurement range.
The advantage of the output current range from 4 to 20 mA is that an output current of less than 4 mA can be detected by a downstream evaluation unit as an error of the transducer or as a broken wire. Of course, it is also possible to choose a different range for the output current, for example, 5 to 20 mA, but an output current range from 4 to 20 mA has prevailed as the industrial standard.
Since modem electrical transducers are generally made as systems-capable intelligent transducers with which both control, and thus error correction of the measured value, as well as communication with an external control and monitoring unit, these electrical transducers usually have a microprocessor as the processor circuit. These processor circuits can process only digital information so that it is necessary for the electrical transducer or the microprocessor to have at least one analog/digital converter and at least one digital/analog converter. The transmission path of these electrical transducers thus consists of an analog sensor, an analog/digital converter, the microprocessor, a digital/analog converter and analog end stage which makes available the output current which is proportional to the quantity to be measured. In these electrical transducers using the two-wire process, the problem is now that, in the least favorable case, only 4 mA is available as a power supply to all electronic components. It follows that conventional, economical microprocessors can be operated only with a short cycle time in order to achieve the required low power consumption of the microprocessor. But this results in that with one such electrical transducer only relatively slow changes of the quantity to be measured can be detected. If fast changes of the quantity to the measured are to be transmitted without significant adulterations, fast and thus power-intensive microprocessors must be used, then the current of 4 mA which is only available in the least favorable case being insufficient.
German Patent DE 16 40 922 C3 discloses the initially described electrical transducer, in which the attempt was made to resolve the contradiction between the requirements for processing speed on the one hand and the power demand of the circuit components on the other, by the transducer having an analog transmission path and a digital transmission path which is located parallel to the latter, which is supplied with the sensor output signal, and into which the processor circuit is inserted. The analog transmission path is used here as the main transmission path for the sensor output signal, while the correction values computed by the processor circuit after conversion into analog signals are combined with the analog output signal of the sensor. In the known electrical transducer, by dividing the transmission path into an analog transmission path and a digital transmission path parallel to it, the speed of response of the transducer to fast changes of the quantity which is to be measured is increased, but in order to accomplish the required low power consumption of the microprocessor, a low clock frequency and thus low processing speed of the processor circuit are necessary.